Precast concrete sanitary manholes are typically comprised of a manhole base, one or more riser sections and top section. The inner diameter of concrete manholes is typically 48 inches or greater. The inner diameter is reduced by use of the top section typically to a standard 24 inch or 30 inch diameter manhole cover normally encountered on city streets and highways. The top concrete section of a manhole is typically a reducing cone or a reducing slab having a wall with a minimum eight inch thickness at the top for load bearing of the cast iron frame which is of a thickness and strength sufficient to accept traffic loading.
Cast iron frames are typically secured to the concrete manhole structure either by being set in a bed of mortar arranged between the top of the top section and the cast iron frame or more frequently are secured to the concrete structure through the use of from 2 to 4 mounting bolts, each typically of a 3/4 inch diameter. Bolts or all-thread rods are always used to secure the cast iron frame to the manhole top section when the manhole is in a flood area necessitating a water-tight mounting.
The standard practice is to set threaded bolt inserts in the top manhole section when it is being cast. The inserts must be set upon exact centers to match the bolt hole centers in the particular cast iron frame. This requires a very precise positioning and holding assembly. In addition, the bolt centers in the cast iron frames are never constant and vary according to the type of cover employed and to the personal preference of the engineers designing the system. As a result, each top concrete manhole section must be manufactured and inventoried for the particular job, requiring an unnecessarily large inventory of both finished sections and mold assemblies.